Microencapsulated controlled-release preparations of nimesulide were formulated. Microparticles were prepared by modified phase separation (non-solvent addition) technique using different ratios of ethylcellulose. The microparticles (M1, M2,and M3) were yellow, free flowing and spherical in shape with the particle size varying from 93.62 ± 14.15 to 104.19 ± 18.15 µm. The t60% of nimesulide release from microparticles was found to be 3 ± 0.6, 5 ± 0.6 and 8 ± 0.8 h for formulations M1, M2,and M3, respectively. FT-IR, XRD, and thermal analysis were done which showed that there is no interaction between the polymer and drug. The mechanism of drug release from nimesulide microparticles was studied by using Higuchi and Korsmeyer-Peppas models. The value of coefficient of determination (R2)for M1, M2,and M3 indicates anomalous and case-II transport release mechanism. The dissolution data of designed system verified its ability to maintain plasma concentration without the need of frequent dosing. The Nimesulide microparticles prolonged drug release for 12 hours or longer. Based on the results of release studies, M3 was opted as a suitable microparticulate formulation allowing the controlled release of nimesulide over a prolonged period of time. Moreover, its encapsulation efficiency was also comparable to the other two formulations (M1 and M2). In conclusion, the influence of polymer concentration should be considered during formulation development.